Instrumentation amplifiers receive differential and common mode input signals and amplify the differential signals. Ideally, the common mode signals are cancelled (rejected) but, due to the non-ideal nature of an actual instrumentation amplifier, the common mode signals are not completely rejected.
Instrumentation amplifiers often contain two input amplifiers, sometimes referred to as positive and negative input amplifiers. The terms positive (processes the non-inverted signal) and negative (processes the inverted signal) are used simply to refer to the different inputs of the instrumentation amplifier. The input amplifiers are ideally identical. The instrumentation amplifier also includes an output section, which is typically an operational amplifier (op amp) that receives the differential and common mode signals from the input amplifiers, removes the common mode, and outputs a voltage corresponding to the difference between the input signals of the instrumentation amplifier. Thus, the instrumentation amplifier cancels (or rejects) signals common to both input signals, such as noise or common mode AC or DC inputs.
The common-mode rejection ratio (CMRR) of a differential amplifier measures the tendency of the device to reject input signals common to both input signals. A high CMRR is important in applications where the differential signal of interest is represented by a small voltage fluctuation superimposed on a possibly large common mode voltage. The CMRR is the ratio of powers of the differential gain over the common-mode gain, expressed in positive decibels.
In an ideal instrumentation amplifier, for near infinite AC CMRR, the positive and negative input amplifiers should be perfectly matched. However, this is practically impossible due to process variations and component tolerances. Consequently, the characteristics of both input amplifiers may change differently over a frequency range.
There are resistors, or other current source elements, in both input amplifiers. The conventional technique to match both input amplifiers is simply to trim one or more of the resistors so that a shorted DC differential input is ideally cancelled at the output. This will result in a zero DC input offset voltage.
Since instrumentation amplifiers are commonly used in precision measuring equipment, the CMRR must be especially good. The inventor has found that the conventional technique of just trimming one or more resistors during a DC test does not optimize the CMRR over a wide range of frequencies, since the bandwidths of the positive and negative input amplifiers may deviate over a range of frequencies. Accordingly, what is needed is an improved technique for adjusting the characteristics of the positive and negative input amplifiers of a precision instrumentation amplifier so that the CMRR remains excellent over a wide frequency range.